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Cosmochemistry and MeteoriticsAn Interview with Dr. John Wasson

John Wasson excavating large mesosiderite meteorites at the Vaca Muerta site in the Atacama Desert of Chile. Image credit: John Wasson.

John Wasson is a professor of chemistry and biochemistry at the University of California at Los Angeles. He holds joint appointments in the Institute of Geophysics and Planetary Physics and two departments - Earth and Space Sciences and Chemistry and Biochemistry. His research interests include cosmochemistry (the chemical composition of the solar system), the solar nebula, and meteoritics. Wasson also has a mineral named after him - Wassonite, composed of sulfur and titanium, which was identified in a meteorite found in Antarctica.

What piqued your interest in planetary science?

While at MIT studying nuclear chemistry I heard lectures by two heroic figures of cosmochemistry, Harold Urey and Hans Suess. By that time I realized I could not master both data-gathering and theory in nuclear chemistry. And my background was similar to that of Suess and Urey. I thought it would be fun to study meteorites and, through this, to learn about the processes that led to the formation of the planets.

You are well known as a cosmochemist with an interest in deciphering nebular and asteroidal fractionation processes. How did your initial interests lead to a focus on the solar nebula and meteoritics?

During the next few years (while I was a postdoc in Munich and a second lieutenant in the Air Force), I read many cosmochemical papers. This led me to a much broader perspective and convinced me that the most important research area I could contribute to was the formation and evolution of the solar nebula.

Artist's conception of the dust and gas surrounding a newly formed planetary system. Image Credit: NASA

What are some of the most significant changes or advancements you have observed in cosmochemistry over the past four decades?

The biggest change is the huge improvements in instrumentation, particularly in the measurement of isotopic ratios, sometimes in very small samples. Another big change has been in the amount of funding available and - because of this - in the increased number of students completing Ph.D. theses in cosmochemistry and of researchers who are full-time cosmochemists.

Your 2005 Whipple lecture focused on impact heating vs. internal heating by short-lived radionuclides in formation processes. Is this still a widely debated topic in current research?

Yes, absolutely. Although there has been some shift in the last decade, the vast majority of cosmochemists still use 26Al heating as their working hypothesis for asteroidal melting even though the chondrites in our museums were formed with 26Al/27Al ratios too low to have produced appreciable melting.

One of your recent projects connected observations of iridium (Ir) in seabed sediments with an estimation of the size of the impacting meteorite that produced the Australasian tektites (natural glass sometimes formed from the impact of a large meteorite). How did you make this determination?

This is hard to describe briefly. Iridium, like several additional noble metals, is an excellent marker of extraterrestrial accretion - something that was emphasized by Alverez et al.’s discovery (1980) of large iridium anomalies in sediments deposited at the Cretaceous-Tertiary (KT) boundary. This is because these elements were largely removed from the Earth's crust and upper mantle during the processes that led to core formation. Our UCLA lab was, and is, good at neutron activation. When new deep-sea cores showed a high abundance of microtektites, we were able to show Ir enhancements in these that, combined with estimates of the radial distribution of the fallout, allowed us to make rough estimates of the total amount of Ir - and thus the mass of the projectile.

The image below shows the location of the Chicxulub Crater as defined by gravity maps. No crater is known for the Australasian tektites. There was probably never a classic bowl-shaped crater there. The impact plume that entrained tektites to Australia and other distant points may have formed at a southeast Asian spot where thousands of small impacts produced a shallow depression that cannot easily be found in topographic studies.

Gravity anomaly map of the Chicxulub impact site associated with major iridium deposits observed at the KT boundary. Image credit: Geological Survey of Canada.

That work involved geology, biology, and chemistry. Do you see cross-disciplinary research as increasingly essential to planetary science? Are there any particular challenges in pursuing cross-disciplinary science?

The determination of the iridium associated with the microtektites didn't involve much biology, but biology was involved in locating the timing of the tektite event relative to the nearest glaciation. Isotopic ratios in shells showed that the Earth was still immersed in a glaciation when the Australasian tektite event occurred, and that it is much easier to make glass when the Earth's surface is covered with loess (the fallout from glaciation-associated dust storms).

Cross-disciplinary research is hard. When scientists come to their views along different paths, they end up basing conclusions on models that involved different working assumptions. I learn a lot from discussions with astrophysicists, but working assumptions commonly get in the way of true collaboration.

Discovered in 2011 within meteorite Yamato 691 enstatite chondrite, the mineral Wassonite was named in recognition of John Wasson for his achievements in meteorite and impact research. Image credit: NASA.

You have quite a prolific publication record, with over 250 publications. Can you share any secrets for balancing extensive publishing with other research responsibilities?

I have been fortunate to have been given much research time by my institution, and I am enough of a maverick that my colleagues haven't wanted me to be a departmental chair. The number of publications doesn't seem so high if you note that the first was published in 1960.

What advice would you give to people beginning or considering a career in planetary science?

Learn basic physics and chemistry and then use your knowledge to challenge authority. Of course, these rules apply anywhere in the sciences. In planetary science we all need to be broad, but we also need to know when it is time to put together a limited package of data and interpretations that is worthy of publication.

If any data were obtainable within our solar system, what process or material samples would would top your most-desired list?

For me, large (>100 g each) sample returns from a large set of asteroids would be the highest priority. The data could then be obtained at leisure in our fine terrestrial labs.

Vesta's south polar region as captured by the Dawn spacecraft on August 12, 2011. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Reading Resources

Here is a collection of further reading resources compiled by Dr. Wasson for those interested in learning more on some of the topics briefly covered in this interview.

Where to Find John Wasson on the Web

Mark Hilverda is a geoscientist and Web Manager for the Planetary Sciences Section. He enjoys being where science, design, and technology collide. Feedback and suggestions for future articles and interviews are always welcome.

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